User equipment apparatus and method for feeding back channel state information in a wireless communication system

ABSTRACT

A method and apparatus are described for transmitting a channel state information (CSI) reporting at a user equipment (UE) in a wireless communication system. A rank indicator (RI) and a first type precoding matrix indicator (PMI) are transmitted to a base station (BS) according to a first CSI feedback type. A second type PMI and a channel quality information (CQI) are transmitted to the BS according to a second CSI feedback type. The RI and the first type PMI are jointly coded, and transmitted through a physical uplink control channel (PUCCH). A reporting period of the first type PMI is longer than a reporting period of the second type PMI. The first type PMI is a wideband PMI, and the second type PMI is a subband PMI. The reporting period of the first type PMI is equal to a reporting period of the RI.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 14/335,675 filed on Jul. 18, 2014, which is a continuation ofU.S. patent application Ser. No. 13/639,062 filed on Oct. 2, 2012 (nowU.S. Pat. No. 8,811,520), which is the National Phase ofPCT/KR2011/002315 filed on Apr. 4, 2011, which claims the benefit ofU.S. Provisional Application Nos. 61/320,324 filed Apr. 2, 2010,61/332,807 filed May 9, 2010, and 61/363,287 filed Jul. 12, 2010, and toKorean Patent Application No. 10-2011-0030168 filed on Apr. 1, 2011, theentire contents of all of the above applications are hereby expresslyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication, and moreparticularly, to a User Equipment (UE) apparatus and method for feedingback Channel State Information (CSI) in a wireless communication system.

2. Discussion of the Related Art

In a cellular Multiple Input Multiple Output (MIMO) communicationenvironment, data rate can be increased through beamforming between atransmitting end and a receiving end. It is determined based on channelinformation whether to use beamforming or not. Basically, the receivingend quantizes channel information estimated from a Reference Signal (RS)to a codebook and feeds back the codebook to the transmitting end.

A brief description will be given of a spatial channel matrix (alsoreferred to simply as a channel matrix) for use in generating acodebook. The spatial channel matrix or channel matrix may be expressedas

${H( {i,k} )} = \begin{bmatrix}{h_{1,1}( {i,k} )} & {h_{1,2}( {i,k} )} & \ldots & {h_{1,{Nt}}( {i,k} )} \\{h_{2,1}( {i,k} )} & {h_{2,2}( {i,k} )} & \ldots & {h_{2,{Nt}}( {i,k} )} \\\vdots & \vdots & \ddots & \vdots \\{h_{{Nr},1}( {i,k} )} & {h_{{Nr},2}( {i,k} )} & \ldots & {h_{{Nr},{Nt}}( {i,k} )}\end{bmatrix}$

where H(i,k) denotes the spatial channel matrix, N_(r) denotes thenumber of Reception (Rx) antennas, N_(t) denotes the number ofTransmission (Tx) antennas, r denotes the index of an Rx antenna, tdenotes the index of a Tx antenna, i denotes the index of an OrthogonalFrequency Division Multiplexing (OFDM) or Single Carrier-FrequencyDivision Multiple Access (SC-FDMA) symbol, and k denotes the index of asubcarrier. Thus h_(r,t)(i,k) is an element of the channel matrixH(i,k), representing the channel state of a t^(th) Tx antenna and anr^(th) Rx antenna on a k^(th) subcarrier and an i^(th) symbol.

A spatial channel covariance matrix R applicable to the presentinvention is expressed as R=E[H(i,k)H^(H)(i,k)] where H denotes thespatial channel matrix, E[ ] denotes a mean, i denotes a symbol index,and k denotes a frequency index.

Singular Value Decomposition (SVD) is one of significant factorizationsof a rectangular matrix, with many applications in signal processing andstatistics. SVD is a generalization of the spectral theorem of matricesto arbitrary rectangular matrices. Spectral theorem says that anorthogonal square matrix can be unitarily diagonalized using a base ofeigenvalues. Let the channel matrix H be an m×m matrix having real orcomplex entries. Then the channel matrix H may be expressed as theproduct of the following three matrices.

H _(m×m) =U _(m×m)ε_(m×n) v _(n×n) ^(H)

where U and V are unitary matrices and ε is an m×n diagonal matrix withnon-negative singular values. For the singular values, ε=diag(σ₁ . . .σ_(r)),σ_(i)=√{square root over (λ_(i))}. The directions of the channelsand strengths allocated to the channel directions are known from the SVDof the channels. The channel directions are represented as the leftsingular matrix U and the right singular matrix V. Among r independentchannels created by MIMO, the direction of an i^(th) channel isexpressed as i^(th) column vectors of the singular matrices U and V andthe channel strength of the i^(th) channel is expressed as σ². Becauseeach of the singular matrices U and V is composed of mutually orthogonalcolumn vectors, the i^(th) channel can be transmitted withoutinterference with a j^(th) channel. The direction of a dominant channelhaving a large σ² value exhibits a relatively small variance over a longtime or across a wide frequency band, whereas the direction of a channelhaving a small σ_(i) ² value exhibits a large variance.

This factorization into the product of three matrices is called SVD. TheSVD is very general in the sense that it can be applied to any matriceswhereas EigenValue Decomposition (EVD) can be applied only to orthogonalsquare matrices. Nevertheless, the two decompositions are related.

If the channel matrix H is a positive, definite Hermitian matrix, alleigenvalues of the channel matrix H are non-negative real numbers. Thesingular values and singular vectors of the channel matrix H are itseigenvalues and eigenvectors.

The EVD may be expressed as

HH ^(H)=(UεV ^(H))(UεV ^(H))^(H) =Uεε ^(H) U ^(H)

H ^(H) H=(UεV ^(H))^(H)(UεV ^(H))=Vεε ^(H) V ^(H)

where the eigenvalues may be λ₁ . . . λ_(r). Information about thesingular matrix U representing channel directions is known from the SVDof HH^(H) and information about the singular matrix V representingchannel directions is known from the SVD of H^(H)H. In general,Multi-User MIMO (MU-MIMO) adopts beamforming at a transmitting end and areceiving end to achieve high data rates. If reception beams andtransmission beams are represented as matrices T and W respectively,channels to which beamforming is applied are expressed asTHW=TU(ε)V^(H)W. Accordingly, it is preferable to generate receptionbeams based on the singular matrix U and to generate transmission beamsbased on the singular matrix V.

The major considerations in designing a codebook are reduction offeedback overhead by using a minimum number of bits and accuratequantization of CSI to achieve a sufficient beamforming gain.

SUMMARY OF THE INVENTION

An object of the present invention devised to solve the problem lies ona method for feeding back Channel State Information (CSI) at a UserEquipment (UE).

Another object of the present invention devised to solve the problemlies on a UE apparatus for feeding back CSI.

It will be appreciated by persons skilled in the art that the objectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and the above and otherobjects that the present invention can achieve will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings.

The objects of the present invention can be achieved by providing amethod for feeding back CSI at a UE in a wireless communication system,including determining a Rank Indicator (RI) for a predeterminedfrequency band, selecting an index of a Precoding Matrix Indicator (PMI)corresponding to the determined RI from a codebook set used for atransmission on the predetermined frequency band, and transmitting theRI and the index of the PMI to a Base Station (BS). The RI and the indexof the PMI are jointly encoded prior to the transmission.

The joint-coded RI and index of the PMI may be transmitted in the samesubframe to the BS.

The joint-coded RI and index of the PMI may be transmitted on a PhysicalUplink Control CHannel (PUCCH) to the BS.

The method may further include receiving information about thepredetermined frequency band from the BS and estimating a channel statebetween the UE and the BS in the predetermined frequency band. The RImay be determined based on the estimated channel state and the index ofthe PMI may be selected based on the estimated channel state.

The predetermined frequency band may be a wideband and thus the PMI maybe a wideband PMI.

In another aspect of the present invention, provided herein is anapparatus for feeding back CSI at a UE in a wireless communicationsystem, including a processor for determining an RI for a predeterminedfrequency band and selecting an index of a PMI corresponding to thedetermined RI from a codebook set used for a transmission on thepredetermined frequency band, and a transmission antenna fortransmitting the RI and the index of the PMI to a BS. The RI and theindex of the PMI are jointly encoded prior to the transmission.

The UE apparatus may further include a reception antenna for receivinginformation about the predetermined frequency band from the BS. Theprocessor may estimate a channel state between the UE and the BS in thepredetermined frequency band, determine the RI based on the estimatedchannel state, and select the index of the PMI based on the estimatedchannel state.

According to various embodiments of the present invention, a UE encodesa Rank Indicator (RI) jointly with a Precoding Matrix Indicator (PMI)and thus transmits the jointly-coded RI and PMI in the same subframe.Therefore, the feedback information is efficiently transmitted.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a block diagram of an evolved Node B (eNB) and a UserEquipment (UE) in a wireless communication system according to thepresent invention.

FIGS. 2 and 3 illustrate exemplary methods for generating a codebook.

FIG. 4 is an exemplary diagram illustrating a signal flow for anoperation for feeding back joint-coded Channel State Information (CSI)at the UE.

FIG. 5 is an exemplary diagram illustrating a signal flow for anoperation for receiving a long-term CSI feedback and a short-term CSIfeedback at the eNB.

FIG. 6 illustrates exemplary CSI feedbacks from the UE.

FIGS. 7A and 7B illustrate exemplary methods for feeding back CSIaccording to different Rank Indicator (RI) values.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the exemplary embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that canbe implemented according to the invention. The following detaileddescription includes specific details in order to provide a thoroughunderstanding of the present invention. However, it will be apparent tothose skilled in the art that the present invention may be practicedwithout such specific details. For example, the following detaileddescription is given under the assumption that a 3^(rd) GenerationPartnership Project Long Term Evolution (3GPP LTE) mobile communicationsystem is being used. However, the description is applicable to anyother mobile communication system except for specific features inherentto the 3GPP LTE system.

In some instances, known structures and devices are omitted, or areshown in block diagram form focusing on important features of thestructures and devices, so as not to obscure the concept of the presentinvention. The same reference numbers will be used throughout thisspecification to refer to the same or like parts.

In the following description, a User Equipment (UE) is assumed to referto a mobile or fixed user end device such as a Mobile Station (MS), anAdvanced Mobile Station (AMS), etc. and the term ‘Base Station (BS)’ isassumed to refer to any node of a network end, such as a Node B, anenhanced Node B (eNB or eNode B), an Access Point (AP), etc.,communicating with a UE.

In a mobile communication system, a UE may receive information from a BSon a downlink and transmit information to the BS on an uplink. Theinformation that the UE transmits or receives includes data and varioustypes of control information. There are many physical channels accordingto the types and usages of information that the UE transmits orreceives.

As examples of a mobile communication system to which the presentinvention is applicable, 3GPP LTE and LTE-Advanced (LTE-A) communicationsystems will be described below.

FIG. 1 is a block diagram of an eNB and a UE in a wireless communicationsystem according to the present invention.

While one eNB 105 and one UE 110 are shown in FIG. 1 to simplify theconfiguration of a wireless communication system 100, the wirelesscommunication system 100 may obviously include a plurality of eNBsand/or a plurality of UEs.

Referring to FIG. 1, the eNB 105 may include a Tx data processor 115, asymbol modulator 120, a transmitter 125, a Transmission/Reception(Tx/Rx) antenna 130, a processor 180, a memory 185, a receiver 190, asymbol demodulator 195, and an Rx data processor 197. The UE 110 mayinclude a Tx data processor 165, a symbol modulator 170, a transmitter175, a Tx/Rx antenna 135, a processor 155, a memory 160, a receiver 140,a symbol demodulator 145, and an Rx data processor 150. While theantennas 130 and 135 are each shown as a single antenna in the BS 105and the UE 110, they include multiple antennas. Hence, the BS 105 andthe UE 110 support Multiple Input Multiple Output (MIMO), specificallyboth Single User-MIMO (SU-MIMO) and Multi User-MIMO (MU-MIMO) in thepresent invention.

On the downlink, the Tx data processor 115 receives traffic data,processes the received traffic data through formatting, coding,interleaving, and modulation (i.e. symbol mapping), and thus outputsmodulated symbols (or data symbols). The symbol modulator 120 processesthe data symbols received from the Tx data processor 115 and pilotsymbols, thus producing a symbol stream.

More specifically, the symbol modulator 120 multiplexes the data symbolsand the pilot symbols and transmits the multiplexed symbols to thetransmitter 125. Each transmission symbol may be a data symbol, a pilotsymbol or a zero signal value. Pilot symbols may be transmittedsuccessively during each symbol period. The pilot symbols may beFrequency Division Multiplexing (FDM) symbols, Orthogonal FrequencyDivision Multiplexing (OFDM) symbols, Time Division Multiplexing (TDM)symbols, or Code Division Multiplexing (CDM) symbols.

The transmitter 125 converts the symbol stream into one or more analogsignals and generates a downlink signal suitable for transmission on aradio channel by additionally processing the analog signals (e.g.amplification, filtering, and frequency upconversion). The downlinksignal is transmitted to the UE 110 through the antenna 130.

The UE 110 receives the downlink signal from the eNB 105 and providesthe received downlink signal to the receiver 140. The receiver 140processes the downlink signal, for example, through filtering,amplification and frequency downconversion and converts the processeddownlink signal to digital samples. The symbol demodulator 145demodulates received pilot symbols and outputs the demodulated pilotsymbols to the processor 155 for use in channel estimation.

The symbol demodulator 145 receives a frequency response estimate of thedownlink from the processor 155 and acquires data symbol estimates (i.e.estimates of the transmitted data symbols) by demodulating the receiveddata symbols using the frequency response estimate. The Rx dataprocessor 150 demodulates the data symbol estimates (i.e. performssymbol demapping), deinterleaves the demodulated data symbols, anddecodes the deinterleaved data symbols, thereby recovering the trafficdata transmitted by the eNB 105.

The operations of the symbol demodulator 145 and the Rx data processor150 are complementary to the operations of the symbol modulator 120 andthe Tx data processor 115 of the eNB 105.

On the uplink, in the UE 110, the Tx data processor 165 outputs datasymbols by processing received traffic data. The symbol modulator 170multiplexes the data symbols received from the Tx data processor 165with pilot symbols, modulates the multiplexed symbols, and outputs astream of the symbols to the transmitter 175. The transmitter 175generates an uplink signal by processing the symbol stream and transmitsthe uplink signal to the eNB 105 through the antenna 135.

The eNB 105 receives the uplink signal from the UE 110 through theantenna 130. In the BS 105, the receiver 190 acquires digital samples byprocessing the uplink signal. The symbol demodulator 195 provides uplinkpilot symbol estimates and uplink data symbol estimates by processingthe digital samples. The Rx data processor 197 processes the data symbolestimates, thereby recovering the traffic data transmitted by the UE110.

The processors 155 and 180 control, adjust and manage operations of theUE 110 and the eNB 105. The processors 155 and 180 may be connectedrespectively to the memories 160 and 185 that store program code anddata. The memories 160 and 185 store an operating system, applications,and general files, in connection with the processors 155 and 180.

The processors 155 and 180 may also be called controllers,microcontrollers, microprocessors, or microcomputers. The processors 155and 180 may be configured in hardware, firmware, software, or acombination of them. When a codebook is generated in hardware accordingto an embodiment of the present invention, Application SpecificIntegrated Circuits (ASICs), Digital Signal Processors (DSPs), DigitalSignal Processing Devices (DSPDs), Programmable Logic Devices (PLDs), orField Programmable Gate Arrays (FPGAs) which are adapted to implementthe present invention may be included in the processors 155 and 180.

On the other hand, if a codebook is generated in firmware or softwareaccording to an embodiment of the present invention, the firmware orsoftware may be configured to include a module, a procedure, a function,etc. which performs functions or operations according to the presentinvention. The firmware or software may be included in the processors155 and 180, or stored in the memories 160 and 185 and invoked from thememories 160 and 185 by the processors 155 and 180.

The layers of radio interface protocols between a UE/eNB and a networkmay be classified into Layers 1, 2 and 3 (L1, L2 and L3) based on thethree lowest layers of the Open System Interconnection (OSI) model. Aphysical layer corresponds to L1 and provides an informationtransmission service on physical channels. A Radio Resource Control(RRC) layer corresponds to L3 and provides radio control resourcesbetween the UE and the network. The UE/eNB and the network exchange RRCmessages through the RRC layers.

One of the codebook design schemes proposed by or approved as recentcommunication standards such as those for mobile communication systems,LTE, LTE-A, Institute of Electrical and Electronics Engineers (IEEE)802.16m, etc. is a hierarchical codebook transformation scheme in whicha long-term Precoding Matrix Indicator (PMI) and a short-term PMI areseparately transmitted and a final PMI is determined using the two PMIs.In a major example of the hierarchical codebook transformation scheme, acodebook is transformed using a long-term covariance matrix of channels,as determined by the following equation, [Equation 1].

[Equation 1]

W′=norm(RW)

where W denotes a conventional codebook representing short-term channelinformation, R denotes the long-term covariance matrix of a channelmatrix H, norm(A) denotes a matrix in which the norm is normalized to 1for each column of a matrix A, and w′ denotes a final codebook achievedby transforming the conventional codebook W using the channel matrix H,the long-term covariance matrix R of the channel matrix H, and the normfunction.

The long-term covariance matrix R of the channel matrix H may be givenas

$\begin{matrix}\begin{matrix}{R = {E\lbrack {H^{H}H} \rbrack}^{(a)}} \\{= {V\; \Lambda \; V^{H}}} \\{= {\sum\limits_{i = 1}^{Nt}\; {\sigma_{i}v_{i}v_{i}^{H}}}}\end{matrix} & \lbrack {{Equation}\mspace{14mu} 2} \rbrack\end{matrix}$

where E[H^(H)H] is decomposed into VAV^(H) in Singular ValueDecomposition (SVD), and σ_(i) and V_(i) are an i^(th) singular value(i.e. an eigenvalue of an i^(th) channel) and an i^(th) singular columnvector corresponding to the i^(th) singular value, respectively (σ₁≧σ₂≧. . . σ_(Nt)). Given a single Tx stream, for example, the codebook W isan N_(t)x1 vector and the transformed codebook w′ satisfies

$W^{\prime} = {\sum\limits_{i = 1}^{Nt}\; {\sigma_{i}{{v_{i}( {v_{i}^{H}W} )}.}}}$

That is, the transformed codebook W′ is determined to be a weightedlinear combination of singular vectors. Herein, the weighted factor ofthe singular column vector v_(i) is determined to be the product of thesingular value σ_(i) and the correlation v_(i) ^(H)W between thesingular column vector v_(i) and the pre-transformation codeword W.

As a consequence, codewords are densely populated around a dominantsingular vector having a large σ_(i), value in the codebook W′, therebyenabling more effective quantization.

FIGS. 2 and 3 illustrate exemplary methods for generating a codebook.

In FIGS. 2 and 3, on the assumption that N_(t) is 2 for the sake ofconvenience, singular vectors and a codebook w are defined in atwo-dimensional space. Although any other codeword distribution ispossible, the codebook w may have a uniform codeword distribution asillustrated in FIG. 2 according to a policy to maximize the minimumdistance between two codewords in a Grassmannian space where channelsexist.

The codebook design policy performs well for uncorrelated channels,while it performs poorly for correlated channels. Moreover, since thecorrelation between a singular vector of instantaneous channels H and asingular vector of a spatial covariance matrix R is high for correlatedchannels, it is effective to adaptively transform the codebook accordingto the spatial covariance matrix R based on the relationship.

FIG. 3 illustrates a transformed codebook. As described before, newcodewords are densely populated around a first dominant singular vectorhaving a large a, value by applying a larger weighting factor to thefirst dominant singular vector. In this manner, as a first dominantsingular vector of the long-term covariance matrix R of the channelmatrix H is weighted with a higher weighting factor, the codebook W′ hascodewords densely populated around the first dominant singular vector,as illustrated in FIG. 3. However, in order to generate a codebookhaving good performance, a minimum distance needs to be maintainedbetween codewords, while the codewords are densely populated around adominant singular vector.

For a high rank, that is, a rank of 2 or higher, the above codebooktransformation scheme faces the problem that the column vectors of thetransformed precoder W′ are not mutually orthogonal. Even thoughorthogonality is maintained between the column vectors of thetransformed precoder W, a non-unitary matrix is multiplied during thetransformation and thus the orthogonality between the column vectors isimpaired after the transformation. When the transformed precoder W′ isused for real implementation, non-orthogonality between beams carryingstreams (represented as the column vectors of transformed precoder W′,)additionally causes inter-stream interference.

Moreover, since energy is distributed to a plurality of channels due tothe increased rank, performance gain is decreased, relative to theconventional non-transformed codebook. For a low rank, codewords aredensely populated in a strong channel direction. In this case, codewordsrepresenting channels accurately can be fed back, compared to codewordsuniformly distributed in the Grassmannian space. On the contrary, for ahigh rank, the channel strength is steered not in a specific directionbut distributed in a plurality of directions. Therefore, codewordsuniformly distributed in the Grassmannian space, that is,pre-transformation codewords can still perform well. This implies thatin case of a high rank, good performance can be achieved even though along-term PMI is fed back with a low granularity using a relativelysmall number of codewords.

Accordingly, it may be preferable to use different codebooks accordingto ranks in calculating a long-term PMI. For a low rank, a long-termcodebook needs to represent a particular channel direction elaboratelyand thus has a large size. For a high rank, a small-sized codebook withunitary matrices may be suitable to ensure orthogonality for thetransformed precoder W′.

The present invention provides a method for changing a long-term PMIcodebook according to a rank to ensure orthogonality for the transformedprecoder W′. A codebook for long-term PMIs, C may include a codebook(i.e., C_(uni)) with unitary codewords and a codebook (i.e., C_(cov))designed for feedback of long-term covariance channel matrices, asexpressed as

[Equation 3]

C=C _(uni) ∪C _(cov),

C _(cov) ={{tilde over (R)} ₀ ,{tilde over (R)} ₁ , . . . ,{tilde over(R)} _(m)},

C _(uni) ={U ₀ ,U ₁ , . . . ,U _(n)}

In [Equation 3], if feedback rank information, that is, an RI is equalto or less than k (k is a positive integer), the long-term PMI codebookmay be limited to the codebook designed for feedback of long-termcovariance channel matrices, C_(cov). If the RI is larger than k, thelong-term PMI codebook may be limited to the codebook with unitarycodewords, C_(uni).

The following description is given in the context that k=1, by way ofexample. If the RI is equal to or larger than 2, the long-term PMIcodebook may be limited to the codebook including unitary matrices only,C_(uni) in order to make the columns of the transformed precoder W′mutually orthogonal. The codebook C_(cov) may include a part or all ofthe codewords of the codebook C_(uni) or may be disjointed from thecodebook C_(uni).

Compared to the codebook C_(cov) that indicates a dominant channeldirection and distributes the codewords of the transformed precoder W′toward the dominant channel direction through combination with thepredefined precoder W, the codebook C_(uni) primarily aims to ensure theorthogonality of the transformed precoder W′ and secondarily aims toincrease the granularity of the unitary codebook by rotating thepredefined precoder W. Therefore, it is typical that the codebookC_(cov) has a large size for accurate feedback and the codebook C_(uni)has a small size relative to the codebook C_(cov). For instance, thecodebook C_(uni) may include only one identity matrix (C_(uni)={I}).

Because one short-term PMI constitutes one perfect precoder in the LTEstandard, control information and a control channel need to be newlydefined for the LTE-A system in which a PMI is divided into a long-termPMI and a short-term PMI. In the legacy LTE system, a UE mayperiodically transmit CSI feedback information such as an RI, a PMI, anda Channel Quality Indicator (CQI) on a Physical Uplink Control CHannel(PUCCH). Due to the limited payload size of the PUCCH, the UE transmitsthe RI, PMI and CQI on the PUCCH in different subframes. Four feedbacktypes are defined according to CSI feedback information. In feedbacktype 3, the RI is fed back in a relatively long period relative to thePMI and the CQI and has a small payload size of up to 2 bits.

A newly added long-term PMI is also fed back in a long period. Thefeedback period of the long-term PMI may be equal to, shorter than, orlonger than the feedback period of the RI. Hereinbelow, feedback typesare proposed for the long-term PMI according to the RI feedback periodand the long-term PMI feedback period according to the presentinvention.

Embodiment 1

In accordance with an embodiment of CSI feedback according to thepresent invention, an RI and a long-term PMI may be fed back in the samefeedback period. In this case, a UE may jointly encode an RI and along-term PMI(e.g, an index of long-term PMI) and feed back thejoint-coded RI and long-term PMI to an eNB. For a maximum rank of r, ajoint-coded codebook C may be given as

[Equation 4]

C=C _(l) ∪C ₂ . . . ∪C _(r),

where

C₁=C_(cov),

C_(i)=C_(uni),

C_(j) is a long term PMI codebook for rank j, (1≦j≦r).

If the RI is 1, the UE selects a codeword from the codebook C₁ and ifthe RI is i other than 1, the UE selects a codeword from the codebookC_(i). Then the UE feeds back the selected codeword to the eNB. Thepayload size of the joint-coded codebook is computed by

$\begin{matrix}{{{ceilling}( {\log_{2}( {\sum\limits_{i = 1}^{r}\; {s( C_{i} )}} )} )},} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

where s(C_(i)) is the number of codewords included in C_(i)

In [Equation 5], the ceiling function, ceiling( ) represents the leastinteger equal to or larger than the solution to an equation included inthe bracket.

For example, if the maximum rank is 4, C_(i)={I}, (2≦i≦4), and s(C₁)=13,a long-term PMI and an RI are jointly encoded to a 4-bit codeword asillustrated in Table 1 below.

TABLE 1 codeword RI and long-term PMI 0000 RI = 1 and long-term PMI = 00001 RI = 1 and long-term PMI = 1 0010 RI = 1 and long-term PMI = 2 0011RI = 1 and long-term PMI = 3 0100 RI = 1 and long-term PMI = 4 0101 RI =1 and long-term PMI = 5 0110 RI = 1 and long-term PMI = 6 0111 RI = 1and long-term PMI = 7 1000 RI = 1 and long-term PMI = 8 1001 RI = 1 andlong-term PMI = 9 1010 RI = 1 and long-term PMI = 10 1011 RI = 1 andlong-term PMI = 11 1100 RI = 1 and long-term PMI = 12 1101 RI = 2 andlong-term PMI = I 1110 RI = 3 and long-term PMI = I 1111 RI = 4 andlong-term PMI = I

Referring to Table 1, I represents an identity matrix. If the RI is 2 orhigher, the UE may configure a long-term PMI codebook with an identitymatrix. Then the UE may jointly encode the RI and a long-term PMI andfeed back the joint-coded RI and long-term PMI to the eNB. In thismanner, the joint coding-based CSI feedback method can be performed fordownlink CSI feedback from a UE to an eNB. In this case, the UE and theeNB may operate in the procedures illustrated in FIGS. 4 and 5,respectively.

FIG. 4 is an exemplary diagram illustrating a signal flow for anoperation for feeding back joint-coded CSI at the UE.

Referring to FIG. 4, the UE may receive a DownLink Reference Signal (DLRS) (S410). While not shown in FIG. 4, the eNB may signal to the UEinformation about a specific frequency band (herein, information aboutthe specific frequency band is previously shared between UE and BS or,the UE may receive information about the specific frequency band fromthe BS) for which the UE is supposed to feed back CSI, or maypreliminarily share the information about the specific frequency bandwith the UE.

The processor 155 of the UE may determine an RI by estimating thedownlink channel state between the UE and the eNB (S420). The processor155 may determine RI assuming transmission on the specific frequencyband in step 420. Specifically, the processor 155 may determine the RIusing a channel quality such as a Signal-to-Interference and Noise Ratio(SINR), the correlation between channels received through multipleantennas, etc. in step S420. For a low rank, tank-1 transmission (i.e.RI=1) is optimal in theory and thus an RI of 1 may be selected. As theSINR increases, a higher rank approximate to a multiplexing gain (i.e.min(N_(t),N_(r))) may be selected. In addition, when antennas are nearto one another or a signal does not scatter much between the UE and theeNB, a channel correlation is high and thus a high rank cannot besupported. Based on this property, the processor 155 of the UEdetermines an optimum rank (i.e. RI) in step S420.

The processor 155 may select a long-term PMI (e.g. an index of awideband Precoding Matrix Indicator (PMI)) corresponding to thedetermined RI from the codebook set used for the transmission on thespecific frequency band (S430). More specially, processor 155 of the UEmay select a PMI (e.g. an index of the PMI) corresponding to thedetermined RI from a codebook set used for the transmission on thespecific frequency band. For example, if the RI is 1, the processor 155may select a codeword that maintains a minimum distance to a long-termcovariance channel matrix from the codebook C_(cov), and if the RI islarger than 1, the processor 155 may select a codeword that maintains aminimum distance to a long-term covariance channel matrix from thecodebook as, as described before with reference to [Equation 4].

The processor 155 jointly encodes the determined RI with the selectedlong-term PMI (S440). To be more specific, the processor 155 jointlyencodes the determined RI with the index of the selected long-term PMIin step S440.

Subsequently, the UE feeds back the joint-coded RI and long-term PMIindex to the eNB (S450). The joint-coded information may be fed back ina long period. The eNB may transmit information about the feedbackperiod to the UE by higher layer signaling. The PMI described in stepsS430 and S440 may be a wideband PMI. The UE may receive informationabout the specific frequency band (e.g. wideband) for determining RI andselecting PMI from the BS (not shown in FIG. 4). In general, a long-termPMI transmitted in a long feedback period is a PMI selected for awideband. The reason for a long-term PMI being a wideband PMI will bedescribed below.

Largely, two types of RSs are defined in a mobile communication systemaccording to their purposes. One type serves the purpose of channelinformation acquisition and the other type is used for datademodulation. For acquisition of downlink channel information at UEs,RSs of the former type need to be transmitted across a wideband and evena UE that does not receive downlink data in a specific subframe shouldbe able to receive and measure these RSs. Such RSs for channelmeasurement may also be used for measurement for handover. RS of thelatter type are transmitted in resources allocated to a downlink signalthat the eNB transmits to the UE. The UE may perform channel estimationusing these RSs of the latter type and thus may demodulate data based onthe channel estimation. Such demodulation RSs should be transmitted in adata transmission region.

Two new types of RSs are largely designed for the LTE-A system, ChannelState Information-Reference Signal (CSI-RS) serving the purpose ofchannel measurement for selection of a Modulation and Coding Scheme(MCS), a PMI, etc. and Demodulation RS (DM-RS) for demodulation of datatransmitted through up to eight Tx antennas.

Compared to conventional CRSs used for both purposes of measurement suchas channel measurement and measurement for handover and datademodulation in the legacy LTE system, CSI-RSs are designed mainly forchannel estimation, although they may also be used for measurement forhandover. Since the CSI-RSs are transmitted only for the purpose ofacquisition of channel information, they may not be transmitted in everysubframe, unlike the CRSs in the legacy LTE system. Accordingly, theCSI-RSs may be configured so as to be transmitted intermittently alongthe time axis, for reduction of CSI-RS overhead. When data istransmitted in a downlink subframe, DM-RSs are also transmitteddedicatedly to the UE for which the data transmission is scheduled.Thus, DM-RSs dedicated to a particular UE may be designed such that theyare transmitted only in a resource area scheduled for the particular UE,that is, only in a time-frequency area carrying data for the particularUE.

As described above, the eNB transmits CSI-RSs intermittently along thetime axis in a long period and that across a wideband for the purpose ofchannel estimation in the LTE-A system. That's why a long-term PMI is awideband PMI.

Referring to FIG. 4 again, as the RI feedback period and the long-termPMI feedback period are same, the UE may feed back the joint-coded RIand selected PMI index to the eNB in the same subframe (S450).

Now a description will be given below of a method for calculating ashort-term PMI and a CQI and feeding back the short-term PMI and CQI tothe eNB by the UE. The processor 155 of the UE first performs codebooktransformation using a long-term PMI codebook according to [Equation 1]to obtain a short-term PMI codebook W. The short-term PMI codebook W maybe determined using a long-term RI as done in conformance to the LTEstandard. After calculating an SINR or a transmission amount based onthe transformed codebook W′, the processor 155 of the UE may calculatean optimum short-term PMI codebook W and a CQI. Because the short-termPMI codebook W and the CQI have a short feedback period, they may betransmitted once or more times within a long-term RI and long-term PMIfeedback period. The processor 155 of the UE may use the latestlong-term RI and long-term PMI in calculating the short-term PMIcodebook W and the CQI.

FIG. 5 is an exemplary diagram illustrating a signal flow for anoperation for receiving a long-term CSI feedback and a short-term CSIfeedback at the eNB.

Referring to FIG. 5, the eNB may receive a long-term RI and a long-termPMI from the UE (S510). That is, the eNB receives the RI and PMI in along feedback period. The eNB may receive a short-term PMI and a CQIfrom the UE (S520) and perform codebook transformation and CSI-basedscheduling (S530). The eNB calculates the codebook W′ through codebooktransformation in the same manner as done at the UE and performsscheduling using the codebook W′ and the CSI feedback (e.g. the CQI andthe RI) received from the UE.

Embodiment 2

In another embodiment of CSI feedback according to the presentinvention, an RI feedback period may be longer than a long-term PMIfeedback period. In this case, the UE transmits an RI and a long-termPMI in different periods. Thus the long-term PMI is fed back at leastonce within the RI feedback period. Like Embodiment 1 in which the RIfeedback period is identical to the long-term PMI period, the long-termPMI (or wideband PMI) is determined according to the RI in Embodiment 2.However, the UE separately encodes the long-term PMI and the RI, ratherthan it jointly encodes them.

If the RI is 1, the UE sets the codebook C_(cov) as a long-term PMIcodebook and feeds back a quantized codeword of long-term covariancechannel matrix selected from the codebook C_(cov) to the eNB. If the RIis larger than 2, the UE sets the codebook C_(uni) as the long-term PMIcodebook and feeds back a codeword selected from the codebook C_(uni) tothe eNB. If C_(uni)={I} and the RI is not 1, the UE does not need tofeed back a long-term PMI because the size of the codebook is 1.Instead, the UE may feed back other information such as a short-term PMIto the eNB in time-frequency resources allocated for transmission of along-term PMI. In this case, the UE and the eNB may operate as follows.

In Embodiment 2, an RI is determined in the same manner as in Embodiment1 in which the RI feedback period and the long-term PMI feedback periodare identical as illustrated in FIG. 4. The processor 155 of the UE maydetermine the RI by estimating the downlink channel state between the UEand the eNB and may select a long-term PMI (or a wideband PMI) accordingto the RI. Then the UE may feed back the determined RI and the selectedlong-term PMI (i.e. the index of the selected long-term PMI) to the eNB.

If the RI is 1, the processor 155 may select a codeword that maintains aminimum distance to a long-term covariance channel matrix from thecodebook C_(cov), and if the RI is larger than 1, the processor 155 mayselect a codeword that maintains a minimum distance to a long-termcovariance channel matrix from the codebook C_(uni). In the latter case,if C_(uni)={I}, which implies that the codebook C_(uni) has only onecodeword, the UE may use given time-frequency resources for anotherusage, instead of feeding back the long-term PMI. For example, the UEmay feedback a short-term PMI in the time-frequency resources.

FIG. 6 illustrates exemplary CSI feedbacks from the UE.

Referring to FIG. 6, when there are a plurality of pieces oftransmittable feedback information in addition to a short-term PMI, theUE also needs to indicate feedback information to the eNB. In FIG. 6,N_(S), N_(L) and N_(R) represent a short-term PMI feedback period, along-term PMI feedback period, and an RI feedback period, respectivelyand the short-term PMI feedback period is equal to a CQI feedbackperiod.

An operation for calculating a short-term PMI and a CQI and then feedingback the short-term PMI and CQI at the UE will first be described below.

The processor 155 of the UE first performs codebook transformation usinga long-term PMI codebook according to [Equation 1] to obtain ashort-term PMI codebook W. The short-term PMI codebook W may bedetermined using a long-term RI as done in conformance to the LTEstandard. After calculating an SINK or a transmission amount based onthe transformed codebook W′, the processor 155 of the UE may calculatean optimum short-term PMI codebook W and a CQI. Because the short-termPMI codebook W and the CQI have a short feedback period, they may betransmitted once or more times within a long-term RI and long-term PMIfeedback period. The processor 155 of the UE may use the latestlong-term RI and long-term PMI in calculating the short-term PMIcodebook W and the CQI. If the RI is larger than 1 and C_(uni){I}, W′=W,which means that the codebook transformation scheme falls back to acodebook non-transformation scheme.

Now a description will be given of an operation for receiving along-term CSI feedback and a short-term CSI feedback from the UE at theeNB. The eNB may receive a long-term RI in a longest feedback period andreceive a long-term PMI in a feedback period shorter than the long-termRI feedback period. One thing to note herein is that for a high rank, along-term PMI can be fixed to one specific value and does not need to befed back. In this case, in given time-frequency resources from the UE,the eNB may receive, instead of the long-term PMI, a short-term PMI anda CQI or compensation factors to mitigate quantization error of CSI.After the eNB receives a short-term PMI and a CQI in a short feedbackperiod from the UE, the eNB may calculate the transformed codebook W′according to the codebook transformation scheme and then performsscheduling based on the codebook W′ and the CSI feedback received fromthe UE.

Embodiment 3

In a further embodiment of CSI feedback according to the presentinvention, an RI feedback period may be shorter than a long-term PMIfeedback period. In this case, an RI may be fed back once or more timeswithin the long-term PMI feedback period. A long-term PMI codebook thatthe UE feeds back can be limited to the codebook C_(cov) regardless ofrank. In this case, if RI is less than predetermined value, a long-termPMI codebook may be used for actual codebook transformation, otherwiseit is not, thus falling back to non-transformation codebook method.

If the UE feeds back downlink to the eNB in this manner, the UE and theeNB may operate as follows, by way of example.

In Embodiment 3, an RI is determined in the same manner as in Embodiment1 in which the RI feedback period and the long-term PMI feedback periodare identical as illustrated in FIG. 4. The processor 155 of the UE maydetermine the RI by estimating the downlink channel state between the UEand the eNB. However, the processor 155 of the UE may select a codewordthat maintains a minimum distance to a long-term covariance channelmatrix from the codebook C_(cov), irrespective of the RI and may feedback the selected codeword to the eNB.

Now a description will be given below of a method for calculating ashort-term PMI and a CQI and feeding back the short-term PMI and CQI tothe eNB by the UE. The processor 155 of the UE first performs codebooktransformation using a long-term PMI according to [Equation 1] to obtaina short-term PMI W. The short-term PMI codebook may be determined usinga long-term RI as done in conformance to the LTE standard. A long-termPMI matrix multiplied by the short-term PMI codebook W is changedaccording to the RI in the following manner.

If the RI is 1, the processor 155 of the UE performs codebooktransformation using the long-term PMI according to the method describedin equation 1. If the RI is larger than 1, the UE just assumes thatlong-term PMI is identity matrix and processor 155 of the UE performscodebook transformation described in equation 1. After calculating anSINR or a transmission amount based on the transformed codeword W′, theprocessor 155 of the UE calculates an optimum short-term PMI W and a CQIbased on the calculated SINR or transmission amount. Because theshort-term PMI W and the CQI have a short feedback period, they may betransmitted once or more times within a long-term RI and PMI feedbackperiod. If the RI is larger than 1 and C_(uni)={I}, W′=W, which meansthat the codebook transformation scheme falls back to a codebooknon-transformation scheme. The processor 155 of the UE calculates anSINR or a transmission amount using the transformed codeword W′, andthen calculate an optimum short-term PMI W and a CQI based on thecalculated SINR or transmission amount. Because the short-term PMI W andthe CQI have a short feedback period, they may be transmitted once ormore times within a long-term RI and long-term PMI feedback period. Theprocessor 155 of the UE may use the latest long-term RI and long-termPMI in calculating the short-term PMI codebook W and the CQI.

The eNB receives the long-term PMI in a longest feedback period from theUE. Herein, the eNB receives one or more RIs within the long-term PMIfeedback period from the UE. The eNB also receives the short-term PMIand the CQI in a short feedback period from the UE. Then the eNBcalculates the transformed codebook W′ in the same manner as done forcodebook transformation at the UE and performs scheduling based on thetransformed codebook W′ and the feedback CSI (e.g. the CQI and the RI).

In the above-described codebook configuration, a long-term PMI codebookis of size N_(t)xN_(t) and a short-term PMI codebook is of size N_(t)xr.Thus a final N_(t)xr codebook is created (r is a rank). Alternatively,an N_(t)xr codebook may be created by multiplying an N_(t)xr PMIcodebook by an rxr PMI codebook. In this case, if the rank is equal toor larger than a predetermined value, an rxr PMI may be limited to anidentity matrix or an rxr PMI may be used by limiting an identity matrixto a certain unitary matrix. For example, an N_(t)xr PMI represents thequantized values of dominant singular vectors v₁, v₂, . . . v_(r), thusbeing an LTE Release 8 codebook.

While it has been described above that a long-term PMI and a short-termPMI are transmitted in different subframes, they may be simultaneouslytransmitted in the same subframe. Especially when the long-term PMI andthe short-term PMI are channel information in the frequency domain, notin the time domain, they may be referred to as a wideband PMI and asubband PMI, respectively. The UE may transmit the wideband PMI and thesubband PMI together to the eNB. If the UE transmits an RI independentlyon a PUCCH in a long period to the eNB, it may transmit three pieces offeedback information together, that is, a long-term PMI (or a widebandPMI), a short-term PMI (or a subband PMI), and a CQI together on thePUCCH in a short period to the eNB. The present invention provides twomethods for effectively transmitting a short-term PMI, a long-term PMI,and a CQI, when a PMI codebook changes in size according to an RI inthis environment.

FIGS. 7A and 7B illustrate exemplary methods for feeding back CSIaccording to different RI values. In FIGS. 7A and 7B, N_(S) and N_(R)represents a short-term PMI feedback period and an RI feedback period.The short-term PMI feedback period is equal to a CQI feedback period anda long-term feedback period.

If the size of a codebook decreases with a higher rank, the payload sizeof a PUCCH carrying a PMI and a CQI may further be reduced for a highrank. For example, if for a rank of 5 or higher, the size of a 8-Txcodebook is fixed to a predetermined value, that is, if the 8-Txcodebook includes only one PMI for a rank of 5 or higher, the UE mayfeed back only a CQI in case of a rank of 5 or higher, as illustrated inFIG. 7A. When the payload size of a control signal is decreasedaccording to a rank in this manner, the Bit Error rate (BER) of the CQIcan be maintained in spite of a low CQI coding rate.

In the LTE system, although channel coding is performed for 11-bit PUCCHpayload, if only a CQI is transmitted, the channel coding is performedfor a 7-bit source (corresponding to 2 codeword transmission) or a 4-bitsource (corresponding to 1 codeword transmission). Therefore, morerobust coding is possible. Alternatively or additionally, for the samecoding rate, transmission power may be decreased. For example, if a7-bit CQI and a 4-bit PMI are allocated to a PUCCH, the PMI is fixed toone value for a predetermined rank, and thus 4 bits out of 11 bits ofthe PUCCH are fixed to certain bits until the next rank information isgenerated, while only 7 bits of the PUCCH are used, the same BER can bemaintained even though the transmission power is decreased to apredetermined level, as illustrated in FIG. 7B. That is, despite thesame coding rate, the same BER can be maintained through powerdeboosting.

Various embodiments have been described in the best mode for carryingout the invention.

A UE apparatus and method for feeding back CSI in a wirelesscommunication system according to the present invention are applicableto mobile communication systems such as 3GPP LTE, LTE-A, and IEEE 802.16systems.

Embodiments described above are combinations of elements and features ofthe present invention. The elements or features may be consideredselective unless otherwise mentioned. Each element or feature may bepracticed without being combined with other elements or features.Further, an embodiment of the present invention may be constructed bycombining parts of the elements and/or features. Operation ordersdescribed in embodiments of the present invention may be rearranged.Some constructions of any one embodiment may be included in anotherembodiment and may be replaced with corresponding constructions ofanother embodiment. It will be obvious to those skilled in the art thatclaims that do not explicitly cite in each other in the appended claimsmay be presented in combination as an exemplary embodiment of thepresent invention or included as a new claim by subsequent amendmentafter the application is filed.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

What is claimed is:
 1. A method for transmitting a channel stateinformation (CSI) reporting at a user equipment (UE) in a wirelesscommunication system, the method comprising: transmitting a rankindicator (RI) and a first type precoding matrix indicator (PMI) to abase station (BS) according to a first CSI feedback type; andtransmitting a second type PMI and channel quality information (CQI) tothe BS according to a second CSI feedback type, wherein the RI and thefirst type PMI are jointly coded, and transmitted through a physicaluplink control channel (PUCCH), and wherein a reporting period of thefirst type PMI is longer than a reporting period of the second type PMI.2. The method of claim 1, wherein the first type PMI is a wideband PMI.3. The method of claim 1, wherein the second type PMI is a subband PMI.4. The method of claim 1, wherein the reporting period of the first typePMI is equal to a reporting period of the M.
 5. The method of claim 1,wherein the second type PMI is calculated based on a latest first typePMI and RI.
 6. A user equipment (UE) for transmitting a channel stateinformation (CSI) reporting in a wireless communication system, the UEcomprising: a transmitter configured to: transmit a rank indicator (RI)and a first type precoding matrix indicator (PMI) to a base station (BS)according to a first CSI feedback type; and transmit a second type PMIand channel quality information (CQI) to the BS according to a secondCSI feedback type, wherein the RI and the first type PMI are jointlycoded, and transmitted through a physical uplink control channel(PUCCH), and wherein a reporting period of the first type PMI is longerthan a reporting period of the second type PMI.
 7. The UE of claim 6,wherein the first type PMI is a wideband PMI.
 8. The UE of claim 6,wherein the second type PMI is a subband PMI.
 9. The UE of claim 6,wherein the reporting period of the first type PMI is equal to areporting period of the RI.
 10. The UE of claim 6, wherein the secondtype PMI is calculated based on a latest reported first type PMI and RI.11. A method for receiving a channel state information (CSI) reportingat a base station (BS) in a wireless communication system, the methodcomprising: receiving a rank indicator (RI) and a first type precodingmatrix indicator (PMI) from a user equipment (UE) according to a firstCSI feedback type; and receiving a second type PMI and channel qualityinformation (CQI) from the UE according to a second CSI feedback type,wherein the RI and the first type PMI are jointly coded, and receivedthrough a physical uplink control channel (PUCCH), and wherein areporting period of the first type PMI is longer than a reporting periodof the second type PMI.
 12. The method of claim 11, wherein the firsttype PMI is a wideband PMI.
 13. The method of claim 11, wherein thesecond type PMI is a subband PMI.
 14. The method of claim 11, whereinthe reporting period of the first type PMI is equal to a reportingperiod of the RI.
 15. The method of claim 11, wherein the second typePMI is calculated based on a latest reported first type PMI and RI. 16.A base station (BS) for receiving a channel state information (CSI)reporting in a wireless communication system, the BS comprising: areceiver configured to: receive a rank indicator (RI) and a first typeprecoding matrix indicator (PMI) from a user equipment (UE) according toa first CSI feedback type; and receive a second type PMI and channelquality information (CQI) from the UE according to a second CSI feedbacktype, wherein the RI and the first type PMI are jointly coded, andreceived through a physical uplink control channel (PUCCH), and whereina reporting period of the first type PMI is longer than a reportingperiod of the second type PMI.
 17. The BS of claim 16, wherein the firsttype PMI is a wideband PMI.
 18. The BS of claim 16, wherein the secondtype PMI is a subband PMI.
 19. The BS of claim 16, wherein the reportingperiod of the first type PMI is equal to a reporting period of the RI.20. The BS of claim 16, wherein the second type PMI is calculated basedon a latest reported first type PMI and RI.